保留两位小数

formatNum(num) {
      let str = ""; //字符串累加
      str = (Math.round(num * 100) / 100)
        .toFixed(2)
        .toString()
        .replace(/(\d)(?=(\d{3})+\.)/g, function($0, $1) {
          return $1 + ",";
        });
      return str; //字符串=>数组=>反转=>字符串
    },